There is a particular kind of disappointment that only a watermelon can deliver. You have watched it swell all summer in the greenhouse border, you cut it on the day the guests arrive, and inside it is pale pink, crunchy and about as sweet as a cucumber. Or you leave it a fortnight too long and the flesh has gone grainy and hollow around the seeds. Between those two failures there is a window of roughly ten days, and everything about getting it right comes down to reading the plant rather than the fruit.
September is when that matters in northern Europe. A watermelon (Citrullus lanatus) sown under glass in March or April and grown on in a greenhouse, polytunnel or against a warm south wall is finishing now, and the plant itself tells you when — if you know which signal to trust. Here are the five checks, ranked by how much weight each one deserves, plus what to do with the fruit once it is off the vine.
- Harvest Late August to late September under cover in northern Europe
- Best single check The tendril beside the fruit stalk: dry, brown and brittle
- Ripens after picking? No — the sugar is fixed at the moment you cut it
- Days from sowing 70–75 for a mini F1 · 85–90 for Charleston Gray
A watermelon does not ripen after you pick it
This is the fact that changes how you approach the whole job. Watermelon is non-climacteric: it has no ripening surge of ethylene once it leaves the plant, so it cannot convert stored starch into sugar the way a tomato or a banana can. The flesh may soften slightly on the kitchen worktop and the colour may deepen a shade, but the sweetness you cut into is exactly the sweetness that was there when you severed the stem. Every gram of sugar in a watermelon arrives through the stalk while the fruit is still attached.
So none of the usual rescues apply. There is no paper bag with a banana in it, no warm windowsill, no equivalent of the routine in our guide to ripening green tomatoes — tomatoes are climacteric and will finish off the plant, watermelons simply will not. Nor is there a curing stage: unlike the pumpkins and winter squash you are lifting at the same time, whose starch keeps converting for weeks in a warm shed, a watermelon cut too early stays exactly as bland as it was.
That makes the decision one-way and worth taking slowly. If you are unsure, wait three days and look again — a watermelon holds on the vine far better than it holds in the kitchen.
The five checks, in order of reliability
1. The tendril. Follow the fruit's stalk back to where it joins the main vine. At that node there is a curly tendril. While the fruit is still filling, it is green and springy; as the fruit finishes, the plant stops supplying it and it dries to a brown, brittle curl. A completely dry tendril is the closest thing to a reliable single indicator that a watermelon offers.
2. The field spot. The patch where the fruit rests on the ground starts out white or pale green. As the melon ripens it turns a deep buttery cream, and on some varieties almost egg-yolk yellow. A white or greenish belly means wait. The same mechanism is at work on winter squash, and our squash harvesting guide explains it in more detail.
3. The skin. Three small things change together. The gloss dulls to a matt, slightly dusty finish; the rind hardens so that a thumbnail no longer dents it; and fine corky brown lines — sugar scarring, or webbing — often appear on the shoulders where sugar has seeped through microscopic splits. Heavy webbing is a good sign.
4. The thump. Rap it firmly with a knuckle. An unripe melon rings tight and high; a ripe one gives a deeper, hollower note as the flesh loosens; an overripe one sounds flat and dead. It is a genuinely useful test on watermelon — more so than on winter squash, where the rind is thick and the cavity small — but it takes a season or two of comparing to calibrate your ear, so treat it as a tiebreaker rather than a verdict.
5. The calendar. Count forward from sowing as a sanity check: 70–75 days for a mini F1 such as Tigrimini, around 80 for Sugar Baby, 85–90 for Crimson Sweet, Janosik and Charleston Gray. From the moment the female flower is pollinated, most varieties need 35 to 45 days. Fruit that set late in July in a cool summer may simply run out of season, which is where cloches, fleece and closed greenhouse vents buy the last fortnight.
No single check is enough on its own. The combination to trust is a dry brown tendril plus a cream-yellow field spot — when both agree, cut.
Checking and cutting, step by step
- Find the tendril first Trace the fruit stalk back to the vine and look at the curl at that node. Green and flexible means wait; brown and crisp means go on to the next check.
- Tip the fruit, do not lift it Ease one side up just far enough to see the field spot. Never twist a watermelon to look underneath — a snapped stalk lets rot straight into the fruit and ends the argument for you.
- Run a thumbnail over the rind If it marks easily the fruit is still growing. A ripe rind resists, has lost its shine and often carries corky brown webbing near the stalk.
- Cut with 5 cm of stem attached Use secateurs or a sharp knife, in the morning while the fruit is cool. Pulling tears the rind at the point where it is most vulnerable, and a stub of stem keeps the wound sealed.
- Rest it somewhere cool, not cold A shaded shed or larder at 10–15 °C is ideal for a week or two. Straight into a cold fridge is the one thing to avoid — see below.
Watermelon or melon? Two fruits, two sets of rules
Watermelon (Citrullus lanatus)
Stays firmly attached until you cut it, and never sweetens off the vine.
- Judge by the dried tendril, the cream field spot and the matt rind
- No smell at the stem end — sniffing tells you nothing at all
- Sugar Baby, Crimson Sweet, Charleston Gray, Janosik and mini Tigrimini F1 in our range
Melon (Cucumis melo)
Climacteric — it does carry on ripening, and it tells you when it is ready.
- The stalk cracks in a ring and the fruit parts from the vine at a touch: "full slip"
- Smells strongly sweet and musky at the stem end when ripe
- Ogen and Orange Pineapple among the Cucumis melo types we stock
It is worth knowing which you are growing before you start prodding, because applying melon logic to a watermelon is the commonest reason people cut too early. Waiting for a watermelon to slip from the vine or to smell of anything will leave you waiting until it splits. Browse the whole Citrullus lanatus range for next spring, or the smaller melon selection if you want a fruit that is rather more forthcoming.
Storing it — and saving the seed
An uncut watermelon keeps for one to two weeks at 10–15 °C. Below about 7 °C it suffers chilling injury: the flesh pits, the rind develops sunken patches and the flavour flattens, which is why a watermelon that has lived in the fridge for a week tastes of very little. Chill it for a few hours before serving by all means, but store it cool rather than cold, in the same way as the roots you are putting away this month. Once cut, wrap the surfaces and refrigerate — three or four days at most. Our guide to harvesting and storing produce covers the rest of the September haul.
A fully ripe fruit is also the only one worth taking seed from. Watermelons are insect-pollinated and cross freely, so seed comes true only if you grew a single variety or hand-pollinated — and never from an F1 such as Tigrimini, whose offspring will be a lottery. Rinse the seed clean of flesh, dry it flat on a plate for a fortnight, and store it somewhere cool and dark; it stays viable for four to five years. The method is the same as for tomato seed minus the fermenting step, and our guides to seed saving and seed viability fill in the detail.
When the plant finally comes out, the greenhouse border it leaves behind is the warmest, best-drained seed bed you own. It is not too late to fill it: parsley sown now will stand there all winter, and there is a full list in what to sow in September. Then give the glass its annual autumn clean.
Common mistakes
- Judging by size. Sugar Baby is ripe at two kilos; Charleston Gray is not ripe until eight or nine. Size tells you about the variety, not the sugar.
- Waiting for a smell. That is a melon test. A ripe watermelon smells of nothing.
- Twisting the fruit to look underneath. A broken stalk is an open wound. Tip it gently instead.
- Leaving too many fruit on one plant. Two well-fed melons beat five small watery ones. Pinch out anything that sets after early August — it will not finish.
- Watering hard in the last three weeks. A sudden drink after a dry spell splits the rind and dilutes the flesh. Ease off as the fruit stops growing; our watering guide explains the steady-supply principle.
Frequently asked questions
How long after flowering is a watermelon ready?
Thirty-five to forty-five days from the female flower opening, depending on variety and how much sun the summer delivered. If you can bear to, tie a loop of wool round the stalk on the day a flower sets — it turns guesswork into arithmetic. Bear in mind the fruit only forms if the flower was visited, so a bee-friendly planting near the greenhouse door earns its keep.
Can I ripen a watermelon indoors?
No. It will soften a little and the colour may deepen slightly, but it will not get sweeter, because the fruit has no starch reserve to convert. This is the single biggest difference between watermelons and almost everything else you harvest in September.
Why did my watermelon split on the plant?
Almost always uneven watering: the flesh swells faster than the rind can stretch after a heavy soak following a dry spell. A split fruit will not keep, so use it the same day. Steady moisture and a mulch prevent it.
Can you really grow watermelons in a northern garden?
Under cover, yes — reliably. Sow indoors in March or April at 22–25 °C, plant into a greenhouse border or a large container in late May, allow two fruit per plant and feed weekly once they set. Outdoors without protection it is a gamble in most seasons. Mini varieties ripen a fortnight earlier and are the safest place to start — see growing from seed in containers and feeding and fertilising.
A watermelon is the one crop in the garden where patience pays a measurable dividend and impatience cannot be undone. Learn the tendril, learn the field spot, and next September you will cut at the right moment without thinking about it. Grow them alongside the cucumbers that want the same warmth and the same greenhouse border, browse our organic vegetable seeds and the full vegetable range, and if this would be your first year under glass, start with the beginner's guide to growing vegetables from seed or work through the rest of our growing guides.
